Dental surface imaging using polarized fringe projection

ABSTRACT

An intra-oral imaging apparatus having a fringe pattern generator energizable to emit a fringe pattern illumination having a predetermined spatial frequency, with light in the approximate 350-500 nm range. A polarizer in the path of the fringe pattern illumination has a first polarization transmission axis. A projection lens is disposed to direct the polarized fringe pattern illumination as incident illumination toward a tooth surface. An imaging lens is disposed to direct light reflected and scattered at the tooth surface along a detection path. An analyzer is disposed along the detection path, having a second polarization transmission axis. A detector disposed along the detection path obtains image data from the light provided through the analyzer. A control logic processor is responsive to programmed instructions and actuable to adjust the intensity over one or more portions of the fringe pattern illumination according to the image data obtained from the detector.

FIELD OF THE INVENTION

The invention relates generally to the field of diagnostic imaging using structured light and more particularly relates to a method for three-dimensional imaging of the surface of teeth and other structures using fringe projection.

BACKGROUND OF THE INVENTION

Fringe projection imaging uses patterned or structured light to obtain surface contour information for structures of various types. In fringe projection imaging, a pattern of lines of an interference fringe or grating is projected toward the surface of an object from a given direction. The projected pattern from the surface is then viewed from another direction as a contour image, taking advantage of triangulation in order to analyze surface information based on the appearance of contour lines. Phase shifting, in which the projected pattern is incrementally spatially shifted for obtaining additional measurements at the new locations, is typically applied as part of fringe projection imaging, used in order to complete the contour mapping of the surface and to increase overall resolution in the contour image.

Fringe projection imaging has been used effectively for surface contour imaging of solid, highly opaque objects and has been used for imaging the surface contours for some portions of the human body and for obtaining detailed data about skin structure. However, a number of technical obstacles have prevented effective use of fringe projection imaging of the tooth. One particular challenge with dental surface imaging relates to tooth translucency. Translucent or semi-translucent materials in general are known to be particularly troublesome for fringe projection imaging. Subsurface scattering in translucent structures can reduce the overall signal-to-noise (S/N) ratio and shift the light intensity, causing inaccurate height data. Another problem relates to high levels of reflection for various tooth surfaces. Highly reflective materials, particularly hollowed reflective structures, can effectively reduce the dynamic range of this type of imaging.

In fringe projection imaging overall, contrast is typically poor, with noise as a significant factor. To improve contrast, many fringe projection imaging systems take measures to reduce the amount of noise in the contour image. In general, for accurate surface geometry measurement using fringe imaging techniques, it is important to obtain the light that is directly reflected from the surface of a structure under test and to reject light that is reflected from material or structures that lie beneath the surface. This is the approach that is generally recommended for 3D surface scanning of translucent objects. A similar approach must be used for intra-oral imaging.

From an optics perspective, the structure of the tooth itself presents a number of additional challenges for fringe projection imaging. As noted earlier, light penetrating beneath the surface of the tooth tends to undergo significant scattering within the translucent tooth material. Moreover, reflection from opaque features beneath the tooth surface can also occur, adding noise that degrades the sensed signal and thus further complicating the task of tooth surface analysis.

One corrective measure that has been attempted to make fringe projection workable for contour imaging of the tooth is application of a coating that changes the reflective characteristics of the tooth surface itself. Here, to compensate for problems caused by the relative translucence of the tooth, a number of conventional tooth contour imaging systems apply a paint or reflective powder to the tooth surface prior to surface contour imaging. For the purposes of fringe projection imaging, this added step enhances the opacity of the tooth and eliminates or reduces the scattered light effects noted earlier. However, there are drawbacks to this type of approach. The step of applying a coating powder or liquid adds cost and time to the tooth contour imaging process. Because the thickness of the coating layer is often non-uniform over the entire tooth surface, measurement errors readily result. More importantly, the applied coating, while it facilitates contour imaging, can tend to mask other problems with the tooth and can thus reduce the overall amount of information that can be obtained.

Even where a coating or other type of surface conditioning of the tooth is used, however, results can be disappointing due to the pronounced contours of the tooth surface. It can be difficult to provide sufficient amounts of light onto, and sense light reflected back from, all of the tooth surfaces. The different surfaces of the tooth can be oriented at 90 degrees relative to each other, making it difficult to direct enough light for accurately imaging all parts of the tooth.

There have been a number of attempts to adapt structured light surface-profiling techniques to the problems of tooth structure imaging. For example, U.S. Pat. No. 5,372,502 entitled “Optical Probe and Method for the Three-Dimensional Surveying of Teeth” to Massen et al. describes the use of an LCD matrix to form patterns of stripes for projection onto the tooth surface. A similar approach is described in U.S. Patent Application Publication 2007/0086762 entitled “Front End for 3-D Imaging Camera” by O'Keefe et al. U.S. Pat. No. 7,312,924 entitled “Polarizing Multiplexer and Methods for Intra-Oral Scanning” to Trissel describes a method for profiling the tooth surface using triangularization and polarized light, but needing application of a fluorescent coating for operation. Similarly, U.S. Pat. No. 6,885,464 entitled “3-D Camera for Recording Surface Structures, In Particular for Dental Purposes” to Pfeiffer et al. discloses a dental imaging apparatus using triangularization but also requiring the application of an opaque powder to the tooth surface for imaging.

It can be appreciated that an apparatus and method that provides accurate surface contour imaging of the tooth, without the need for applying an added coating or other conditioning of the tooth surface for this purpose, would help to speed reconstructive dentistry and could help to lower the inherent costs and inconvenience of conventional methods, such as those for obtaining a cast or other surface profile for a crown, implant, or other restorative structure.

SUMMARY OF THE INVENTION

It is an object of the present invention to advance the art of diagnostic imaging, particularly for intra-oral imaging applications. With this object in mind, the present invention provides an intra-oral imaging apparatus comprising: a fringe pattern generator energizable to emit a fringe pattern illumination having a predetermined spatial frequency, with light in the 350-500 nm range; a polarizer in the path of the fringe pattern illumination emitted from the fringe pattern generator and having a first polarization transmission axis; a projection lens disposed to direct the polarized fringe pattern illumination as incident illumination toward a tooth surface; an imaging lens disposed to direct at least a portion of the light reflected and scattered from the incident illumination at the tooth surface along a detection path; an analyzer disposed along the detection path and having a second polarization transmission axis; a detector disposed along the detection path for obtaining image data from the light provided through the analyzer; and a control logic processor responsive to programmed instructions and actuable to obtain image data from the detector and to adjust the intensity over one or more portions of the fringe pattern illumination that is emitted from the fringe pattern generator according to the obtained image data.

It is a feature of the present invention that it applies light of suitable polarization and wavelength along with fringe projection patterning of varying brightness to the task of tooth contour imaging.

An advantage offered by the apparatus and method of the present invention relates to improved imaging of tooth surfaces and at lower cost over conventional contour imaging methods. Unlike conventional methods, no powder or other opaque substance must be applied to the tooth as a preparatory step for contour imaging.

These objects are given only by way of illustrative example, and such objects may be exemplary of one or more embodiments of the invention. Other desirable objectives and advantages inherently achieved by the disclosed invention may occur or become apparent to those skilled in the art. The invention is defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the embodiments of the invention, as illustrated in the accompanying drawings. The elements of the drawings are not necessarily to scale relative to each other.

FIG. 1 is a schematic diagram of an imaging apparatus using polarized fringe projection imaging in one embodiment.

FIG. 2A is a block diagram showing the use of an analyzer with its polarization axis in parallel to the polarizer of a polarized fringe projection imaging apparatus.

FIG. 2B is a block diagram showing the use of an analyzer with its polarization axis orthogonal to the polarizer of a polarized fringe projection imaging apparatus.

FIG. 3A shows the polarization-dependent reflection and scattering of illumination incident on the tooth.

FIG. 3B is a diagram showing the relative intensities of reflected light and the scattered light from incident illumination.

FIGS. 4A, 4B, and 4C are perspective views of a tooth imaged with fringe projection imaging, using non-polarized light, cross-polarized light, and co-polarized light, respectively.

FIG. 5A is a diagram showing wavelength-dependent penetration of illumination incident on the tooth.

FIG. 5B is a schematic diagram showing relative intensities of reflected and scattered light with different wavelengths.

FIG. 6 is a schematic diagram showing an imaging apparatus for obtaining both co-polarized and cross-polarized light in fringe projection imaging.

FIG. 7 is a block diagram showing components of an intra-oral imaging system according to one embodiment.

FIG. 8 is a schematic diagram showing how increased brightness can be applied for improved imaging over a portion of the imaging field with contoured surfaces.

FIGS. 9A and 9B show exemplary projected light patterns generated for contour imaging in one embodiment.

FIG. 10 is a logic flow diagram that shows the sequence for obtaining a contour-compensated image.

FIG. 11 is a schematic block diagram showing components of a pattern generator in one embodiment.

FIG. 12 is a schematic diagram of an imaging apparatus using polarized fringe projection imaging in one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The figures provided herein are given in order to illustrate key principles of operation and component relationships along their respective optical paths according to the present invention and are not drawn with intent to show actual size or scale. Some exaggeration may be necessary in order to emphasize basic structural relationships or principles of operation. Some conventional components that would be needed for implementation of the described embodiments, such as support components used for providing power, for packaging, and for mounting and protecting system optics, for example, are not shown in the drawings in order to simplify description of the invention itself. In the drawings and text that follow, like components are designated with like reference numerals, and similar descriptions concerning components and arrangement or interaction of components already described are omitted.

In the context of the present disclosure, the term “fringe pattern illumination” is used to describe the type of structured illumination that is used for fringe projection imaging or “contour” imaging. The fringe pattern itself can include, as pattern features, multiple lines, circles, curves, or other geometric shapes that are distributed over the area that is illuminated and that have a predetermined spatial frequency, recurring at a given period.

Two portions of a line of light or other feature in a pattern of structured illumination can be considered to be substantially “dimensionally uniform” when their line width is the same over the length of the line to within no more than +/−15 percent. As is described in more detail subsequently, dimensional uniformity of the pattern of structured illumination is needed to maintain a uniform spatial frequency.

As noted above in the background section, conventional approaches for fringe projection imaging fall short of providing good results for tooth tissue for a number of reasons. Apparatus and methods of the present invention address the problems of obtaining images of the tooth when using fringe projection imaging with fringe pattern illumination by selection of favorable light properties and by techniques that improve light delivery to the highly contoured tooth surface.

Referring to the schematic block diagram of FIG. 1, there is shown an embodiment of an intra-oral imaging apparatus 10 for obtaining surface contour information from a tooth 20 using structured light. A fringe pattern generator 12 is energizable to form the structured light as a fringe pattern illumination and project the structured light thus formed as incident light toward tooth 20 through a polarizer 14 and projection lens 16. Light reflected and scattered from tooth 20 is provided to a detector 30, through an imaging lens 22 and an analyzer 28. Detector 30 is disposed along a detection path 88, at the image plane of imaging lens 22. A control logic processor 34 accepts feedback information from detector 30 and, in response to this and other data, is actuable to effect the operation of pattern generator 12, as described in more detail subsequently.

One function of control logic processor 34 for fringe projection imaging is to incrementally shift the position of the fringe and trigger the detector to take images that are then used to calculate three-dimensional information of tooth surface. For the phase shifting fringe projection method, at least three images are typically needed in order to provide enough information for calculating the three-dimensional information of the object. The relative positions of the fringes for these three projected images are typically shifted by one-third of the fringe period. Control logic processor 34 can be a computer, microprocessor, or other dedicated logic processing apparatus that executes programmed instructions.

Intra-oral imaging apparatus 10 of FIG. 1 uses polarized light for surface imaging of tooth 20. Polarizer 14 provides the fringe pattern illumination from fringe pattern generator 12 as linearly polarized light. In one embodiment, the transmission axis of analyzer 28 is parallel to the transmission axis of polarizer 14. With this arrangement, only light with the same polarization as the fringe pattern is provided to the detector 30. In another embodiment, analyzer 28, in the path of reflected light to detector 30, is rotated by an actuator 18 into either of two orientations as needed:

-   -   (a) Same polarization transmission axis as polarizer 14. In this         “co-polarization” position, detector 30 obtains the specular         light reflected from the surface of tooth 20, and most of the         light scattered and reflected from the superficial layer of         enamel surface of tooth 20, as well as some of the light         scattered back from sub-surface portions of the tooth. The         co-polarization orientation of the analyzer 28 axis is shown in         FIG. 2A. Parallel or co-polarization provides improved contrast         over other configurations.     -   (b) Orthogonal polarization transmission axis relative to         polarizer 14. Using the orthogonal polarization, or         cross-polarization, helps to reduce the specular component from         the tooth surface and obtain more of the scattered light from         inner portions of the tooth. The cross-polarization orientation         of the analyzer 28 axis is shown in FIG. 2B.

When the tooth is imaged with an imaging system and sensor, the light that is available to the sensor can be (i) light reflected from the tooth top surface; (ii) light scattered or reflected from the near surface volume or portion of the tooth; and (iii) light scattered inside the tooth. In the context of the present disclosure, the “near-surface volume” of the tooth is that portion of the tooth structure that lies within no more than a few hundred μm of the surface.

It is known that the light reflected from the tooth surface (i), the specular light, maintains the polarization state of the incident light. As the incident light propagates further into the tooth, the light is increasingly depolarized.

Disadvantageously, some portion of the specular light (i) for a contour pattern may be incident on more highly reflective portions of the tooth surface, even causing some amount of saturation that degrades light detection. In contrast to conventional approaches that use all the light from the tooth, methods of the invention use at least portions of both the specular light (i) and the near-surface reflected light (ii), and avoid the light scattered deep inside the tooth (iii). Applicants have found that the near-surface light (ii), particularly for blue light and shorter wavelengths, is still substantially polarized. Thus, for example, a large portion of the light scattered and reflected from the superficial layer of the tooth enamel also has the same polarization state as the incident light and as the specular light (i).

FIG. 3A shows why the apparatus and method of the present invention use scattered near-surface light from just beneath the surface of the tooth. When a polarized light P0 with small dimension illuminates the tooth, some of the light P1 is reflected from the surface of the tooth in specular fashion and has the same polarization state as the illumination light P0. The other portion of the illumination light P0 goes into the tooth, is subject to scattering and depolarizes. Some of the scattered light P2 escapes the tooth surface near the illumination region and can reach detector 30 (FIG. 1).

Of particular interest, the spatial “footprint” of the scattered light P2, which relates to the dimensions of pattern features of the structured light, such as line thicknesses, shows an increase over the corresponding spatial footprint of reflected light P1. For example, where the structured light pattern consists of parallel lines of light of a given thickness, the reflected light P1 from these pattern features has lines of substantially the same thickness as the projected pattern. However, the scattered light P2 is detected as lines of slightly increased thickness. That is, since light P2 has been scattered inside the tooth, the projected footprint on the tooth surface is broader than that of the specular reflected light, which is the same size as the illumination beam. The graph of FIG. 3B shows the difference between the footprint of the light from the tooth surface (P1) and the light from inside the tooth (P2). To reduce the measurement error that can result, the light detected from inside the tooth should be minimized. Applicants have found that polarization provides an effective discriminator for separating the specular light (P1) from the tooth surface from the scattered light from inside the tooth, while still taking advantage of a portion of the scattered light (P2).

The group of contour images shown in FIGS. 4A-4C gives a comparison of approaches for obtaining and using light returned from the tooth using fringe projection. FIG. 4A shows a contour image of tooth 20 obtained using unpolarized light. FIG. 4B shows a somewhat poorer image using cross-polarized light, but not exhibiting specular reflection. FIG. 4C shows the improvement in the image contrast when using co-polarized light. Areas of high brightness in this image are due to specular reflection. As these images show, fringe contrast improves when the cross-polarization light is blocked from the image detector.

In addition to taking advantage of favorable properties of polarized light, embodiments of the present invention also take advantage of different amounts of reflection that correspond to the wavelength of light directed toward the tooth. FIG. 5A shows three different wavelengths λ1, λ2, and λ3 as directed toward tooth 20. The shortest wavelength at λ1 penetrates the tooth the shortest distance. The next longest wavelength at λ2 penetrates the tooth an additional distance. Finally, the longest wavelength at λ3 penetrates the tooth the farthest distance. The graph of FIG. 5B shows how scattering affects the footprint of the light on the tooth surface from each wavelength. The longer the wavelength, the larger the footprint, resulting in larger measurement error. Wavelength λ1 could be near-UV or blue light in the range of 350 to 500 nm, for example. Wavelength λ2 could be green light in the range of 500 to 700 nm, for example. Wavelength λ3 could be red or IR light in the range of 700 nm or higher, for example. Thus, blue or near UV light in the approximate 350-500 nm range, because it provides the least penetration into the tooth structure, proves to be a suitable light source for fringe projection imaging in one embodiment.

For the embodiment of FIG. 1, spatial light modulators can be used as part of fringe pattern generator 12 to provide the needed shifting motion for polarized fringe projection imaging, as described in more detail subsequently. The fringe pattern itself is shifted to at least one alternate position during imaging, more preferably to two or more alternate positions. This shifting of the light pattern can be caused by a separate actuator (not shown in FIG. 1), such as a piezoelectric or other type of actuator that is part of fringe pattern generator 12 for achieving precision incremental movement. Alternately, where fringe pattern generator 12 uses a spatial light modulator, this shifting can be performed electronically, without mechanical movement of parts within fringe pattern generator 12. In addition, another actuator 18 can be positioned for providing 90 degree rotation to either polarizer 14 or analyzer 28 (such as is shown in FIG. 1) in order to obtain both co-polarization and cross-polarization images. Polarization can also be rotated when using an LCD spatial light modulator.

FIG. 6 shows an embodiment of an intra-oral imaging apparatus 40 that obtains images using both parallel and cross-polarization without requiring rotation of either polarizer 14 or analyzer 28 between image captures. A polarization beam splitter 36 separates the reflected and scattered light, reflecting the cross-polarized light to a detector 30 b and transmitting the co-polarized light to a detector 30 a.

Because the co-polarized and cross-polarized light provide different types of information about the surface and near-surface of the tooth, imaging apparatus 40 of FIG. 6 offers the advantage of using both polarizations without the need for mechanical movement of analyzer 28 or polarizer 14, combining the results from orthogonal polarizations in order to obtain improved surface contour data.

Detectors 30, 30 a, or 30 b in the embodiments described herein can be any of a number of types of image sensing array, such as a CCD device, for example. Polarizers and analyzers can be wire-grid or other polarizer types.

In one embodiment of the present invention, the imaging apparatus is packaged in the form of a hand-held probe that can be easily positioned within the patient's mouth with little or no discomfort. Referring to FIG. 7, there is shown an intra-oral imaging system 42 that includes imaging apparatus 10 in the form of a probe. The probe communicates, over a wired or wireless data communication channel, with control logic processor 34 that obtains the images from either or both co-polarized and cross-polarized projection fringes. Control logic processor 34 provides output image data that can be stored as a data file and displayed on a display 38.

As noted in the background section, the pronounced contours of the tooth include surfaces that are steeply sloped with respect to each other, complicating the task of directing enough light onto each surface. As a result, some surfaces of the tooth may not provide 3-D information that is sufficient. Referring to FIG. 8, this problem is represented relative to a rear surface 26 of tooth 20. Patterned light from imaging apparatus 10 generates a contour-detecting fringe pattern 44 onto tooth 20, as shown in box B. Fringe pattern 44 is sufficiently bright for obtaining 3-D image content over a top surface area, as outlined over an area 52; however, the back surface area corresponding to rear surface 26 of tooth 20 and outlined as a darker area 54 is very dimly lit. This allows only a coarse estimation, at best, of the contour of rear surface 26.

In order to compensate for this lack of brightness using conventional fringe projection patterning techniques, an embodiment of the present invention selectively increases the light intensity of the fringe pattern illumination over a given area. In FIG. 8, a fringe pattern 50 is shown with two different areas, differentiated by their relative light intensities. In fringe pattern 50, a first intensity 56 is provided for fringe projection imaging of surfaces such as top surface area that are more readily accessible for contour imaging. A second intensity 58, higher than first intensity 56 for the example shown and as indicated by darker lines in FIG. 8, is provided for the back surface area of the tooth. It should be observed that the actual pattern feature spacing and thickness of the projected contour lines that are the pattern features in this example is not changed in this embodiment. The same spatial frequency of fringe pattern 50 is preserved. This means that the contour pattern, fringe pattern 50, remains dimensionally uniform, with individual lines or other pattern features changed only in intensity, rather than in dimension or spacing (period). Only the relative intensity of the fringe pattern illumination over one or more areas is increased where needed. For example, along any one line within structured light fringe pattern 50, there can be any number of intensities, such as the two shown as first and second intensities 56 and 58 in FIG. 8. The line thickness within the fringe pattern does not change; the spatial frequency of the fringe pattern is preserved.

Maintaining dimensional uniformity and spatial frequency of the fringe pattern is advantageous for contour imaging because it provides a uniform resolution over the full image field. Other techniques have been proposed for changing the pattern dimensions itself, such as thickening the pattern lines over specific areas; however, because the spatial frequency of the fringe pattern changes when using such a technique, the resulting resolution of the contour image that is obtained is non-uniform. With respect to the example fringe pattern 50 given in FIG. 8, it is instructive to observe that if the area indicated as second intensity 58 actually used thicker lines, the resulting contour image would suffer reduced resolution over this area. By maintaining the lines of fringe pattern 50 as dimensionally uniform and only increasing the intensity of light to provide second intensity 58 in this example, embodiments of the present invention provide an increased illumination without loss of resolution over the darker region.

The schematic diagram of FIG. 8 showed a simple case in which fringe pattern 50 compensates for surface steepness by using two different intensities 56 and 58. FIGS. 9A and 9B show examples of other possible arrangements that use more than two light intensities. In FIG. 9A, for example, light for the fringe pattern illumination can be of first intensity 56, second intensity 58, or a third intensity 66, represented as the highest intensity in this example. In FIG. 9B, light can be of first, second, or third intensities 56, 58, or 66 respectively, or of an even higher fourth intensity 68 as shown. The light intensity can vary along any individual pattern feature, such as along a single line in the projected fringe pattern 50.

In addition to increasing the light intensity over darker areas of the tooth surface relative to the position of imaging apparatus 10, it is also possible to reduce the light intensity over areas where there may be highly specular reflection that otherwise causes saturation of the detector. Again, it must be emphasized that what changes is the light intensity over one or more portions of the projected light pattern; line thickness and spacing, both related to the spatial frequency, remain the same for different intensities.

Referring again to FIGS. 1 and/or 6, the light intensity over the projected pattern can be changed by controlling fringe pattern generator 12 by means of commands from control logic processor 34, in response to programmed instructions, and by means of signals provided from control logic processor 34 to related control components. In one embodiment, fringe pattern generator 12 is a digital micromirror device (DMD). Intensity can then be increased over any portion of projected fringe pattern 50 by increasing the effective duty cycle of the rotatable mirrors of the DMD using Pulse-Width Modulation (PWM), so that the source illumination is provided for a suitable amount of time over a particular portion of the fringe pattern. Other methods of illumination intensity adjustment would apply for LCD and for other transmissive and emissive spatial light modulators, using light modulation techniques familiar to those skilled in the imaging arts.

Referring again to FIG. 7, control logic processor 34 is programmed with instructions that automatically adapt the local intensities of lines or other features in fringe pattern 50 according to imaging conditions.

The logic flow diagram of FIG. 10 shows a sequence of steps that are used for adaptive fringe projection imaging in one embodiment. In an initial step 60 a first reference image is obtained. The reference image can be a contour image, formed by projecting structured light onto the tooth surface. Alternately, the reference image can be a conventional two-dimensional image obtained from projection of a uniform field of light onto the tooth surface. The reference image that is obtained can be at full resolution; alternately, since the reference image is not used directly for imaging but instead to determine the overall amount of light that is returned over each surface area, the reference image can be at lower resolution.

Still referring to FIG. 10, an analysis step 64 follows, in which areas from the sensed reference image that are not sufficiently bright are identified. For dental imaging applications, analysis step 64 can take advantage of known data about tooth structure. The operator, for example, may identify the tooth by number or provide other information that is used in analysis step 64. A map generation step 70 is then executed, in which areas of greater or lesser intensity are defined according to the first reference image. With respect to FIGS. 9A and 9B, step 70 then sets up variable intensity fringe pattern 50. An image acquisition step 74 then uses the generated fringe pattern 50 for obtaining a contour image with added brightness as described with respect to FIG. 8. Image acquisition step 74 may be followed by an optional looping step 76 that repeats the analysis of map generation step 70 in order to generate a second or other additional mappings so that the projected structured illumination pattern can be shifted, with appropriate changes in intensity, one or more times. This shifting is done in order to obtain a more accurate evaluation of tooth contour using fringe projection techniques. The individually obtained contour images are combined to obtain surface structure information, using techniques well known in the imaging arts. In one embodiment, image acquisition step 74 also includes energizing actuator 18 (FIG. 1) in order to obtain images using both co-polarization (as in FIG. 2A) and cross-polarization (FIG. 2B).

FIG. 11 is a schematic block diagram showing components of fringe pattern generator 12 in one embodiment. A spatial light modulator 84, such as a digital micromirror device (DMD), liquid crystal device (LCD), or other type of light modulator array or grating forms a pattern according to control signals from control logic processor 34. A light source 80 provides incident light to spatial light modulator 84, conditioned by one or more optical elements 82, such as a light uniformizer and lens elements. Spatial light modulator 84 in this embodiment may be a transmissive device as shown in FIG. 11 or a reflective device, such as a DMD. Control logic processor 34 responds to pattern 44 of light brightness that is returned in the initial reference image as was described earlier with reference to FIG. 8 to control the intensity of pattern features in the fringe pattern that it forms on spatial light modulator 84.

In the embodiment shown in FIG. 11, light source 80 can be a solid-state light source, such as a Light-Emitting Diode (LED) or laser, or can be a lamp or other light source. Blue or near UV light in the 350-500 nm range is used for providing usable image content from near-surface portions of the tooth, as described earlier. In an alternate embodiment, light source 80 is not used and an emissive array, such as an Organic LED (OLED) is used for pattern generation from a single component.

The schematic diagram of FIG. 12 shows another embodiment of the present invention wherein a filter 90, such as a bandpass filter that transmits blue or near UV light in the 350-500 nm range and attenuates other light, is placed in the imaging path. This embodiment can be less sensitive to factors in the environment, such as stray light from other equipment in the room. In this embodiment, light source 80 within fringe pattern generator 12 (FIG. 11) can be either broadband, extending well beyond the 350-500 nm range, or narrow-band, primarily emitting blue and near-UV light.

Embodiments of the present invention provide improved contour imaging for teeth by taking advantage of properties of light and capabilities of spatial light modulators for forming an adaptive fringe projection pattern having suitable light intensity that is responsive to variability in tooth surface characteristics. The apparatus and methods of the present invention compensate for problems related to the translucence of the tooth by using short-wavelength light and by employing principles of polarized light. When light of suitable wavelength and polarization state is provided with an adaptable intensity arrangement, a more accurate indicator of the highly contoured tooth surface can be achieved.

The surface contour image that is obtained using the apparatus and methods of the present invention can be used in a number of ways. Contour data can be input into a system for processing and generating a restorative structure or can be used to verify the work of a lab technician or other fabricator of a dental appliance. This method can be used as part of a system or procedure that reduces or eliminates the need for obtaining impressions under some conditions, reducing the overall expense of dental care. Thus, the imaging performed using this method and apparatus can help to achieve superior fitting prosthetic devices that need little or no adjustment or fitting by the dentist. From another aspect, the apparatus and method of the present invention can be used for long-term tracking of tooth, support structure, and bite conditions, helping to diagnose and prevent more serious health problems. Overall, the data generated using this system can be used to help improve communication between patient and dentist and between the dentist, staff, and lab facilities.

Advantageously, the apparatus and method of the present invention provide an intra-oral imaging system for 3-D imaging of teeth and other dental features without requiring the use of a special powder or application of some other temporary coating for the tooth surface. The system offers high resolution, in the 25-50 μm range in one embodiment.

The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. For example, any of a number of different types of spatial light modulator could be used as part of the fringe pattern generator. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.

PARTS LIST

-   10. Imaging apparatus -   12. Fringe pattern generator -   14 Polarizer -   16. Lens -   18. Actuator -   20. Tooth -   22. Lens -   26. Rear surface -   28. Analyzer -   30, 30 a, 30 b. Detector -   34. Control logic processor -   36. Polarization beam splitter -   38. Display -   40. Imaging apparatus -   42. Intra-oral imaging system -   44. Pattern -   50. Fringe pattern -   52, 54. Area -   56. First intensity -   58. Second intensity -   60. Initial step -   64. Analysis step -   66. Third intensity -   68. Fourth intensity -   70. Map generation step -   74. Image acquisition step -   76. Looping step -   80. Light source -   82. Optical element -   84. Spatial light modulator -   88. Detection path -   90. Filter -   B. Box -   P0, P1, P2. Polarized light -   λ1, λ2,λ3. Wavelength 

1. An intra-oral imaging apparatus comprising: a fringe pattern generator energizable to emit a fringe pattern illumination having a predetermined spatial frequency, with light in the approximate 350-500 nm range; a polarizer disposed in the path of the fringe pattern illumination emitted from the fringe pattern generator and having a first polarization transmission axis; a projection lens disposed to direct the polarized fringe pattern illumination as incident illumination toward a tooth surface; an imaging lens disposed to direct at least a portion of the light reflected and scattered from the incident illumination at the tooth surface along a detection path; an analyzer disposed along the detection path and having a second polarization transmission axis; a detector disposed along the detection path for obtaining image data from the light provided through the analyzer; and a control logic processor responsive to programmed instructions and actuable to obtain image data from the detector and to adjust the intensity over one or more portions of the fringe pattern illumination that is emitted from the fringe pattern generator according to the obtained image data.
 2. The imaging apparatus of claim 1 wherein the second polarization transmission axis is parallel to the first polarization transmission axis.
 3. The imaging apparatus of claim 1 wherein the second polarization transmission axis is orthogonal to the first polarization transmission axis.
 4. The imaging apparatus of claim 1 further comprising an actuator coupled to either the polarizer or the analyzer and energizable to rotate the coupled polarizer or analyzer to one of two positions, substantially 90 degrees apart.
 5. The imaging apparatus of claim 1 wherein the analyzer is a polarization beam splitter and wherein the detector is a first detector disposed to receive light transmitted through the polarization beam splitter and further comprising a second detector disposed to receive light reflected from the polarization beam splitter.
 6. The imaging apparatus of claim 1 wherein the fringe pattern generator comprises a grating.
 7. The imaging apparatus of claim 1 wherein the fringe pattern generator comprises a spatial light modulator.
 8. The imaging apparatus of claim 7 wherein the spatial light modulator is taken from the group consisting of a digital micromirror device and a liquid crystal device.
 9. The imaging apparatus of claim 1 wherein the fringe pattern generator comprises an emissive device that forms and emits the fringe pattern.
 10. The imaging apparatus of claim 1 further comprising a filter disposed along the detection path for transmitting light in the 350-500 nm range.
 11. A method for obtaining a surface image of a tooth comprising: obtaining a reference image of a portion of the tooth surface having at least a lighter area and a darker area; generating and projecting one or more fringe patterns onto the portion of the tooth surface, wherein the intensity of the fringe pattern illumination that is directed toward the darker area exceeds the intensity of the fringe pattern illumination that is directed toward the lighter area and wherein the fringe pattern has the same spatial frequency for pattern features over the lighter and darker areas; and obtaining one or more images from the one or more projected fringe patterns.
 12. The method of claim 11 wherein obtaining the reference image further comprises directing a structured illumination onto the tooth surface.
 13. The method of claim 11 wherein projecting the one or more fringe patterns comprises modulating light of a wavelength in the range from 350-500 nm.
 14. The method of claim 11 wherein projecting the one or more fringe patterns comprises modulating polarized light.
 15. The method of claim 11 wherein obtaining the reference image further comprises changing the transmission axis of an analyzer or a polarizer.
 16. The method of claim 11 wherein generating the one or more fringe patterns comprises actuating a spatial light modulator.
 17. The method of claim 11 wherein obtaining the reference image comprises obtaining light directed from a polarization beamsplitter.
 18. The method of claim 11 wherein generating the one or more fringe patterns further comprises shifting the relative spatial position of two or more of the generated fringe patterns.
 19. The method of claim 18 wherein shifting the relative spatial position of two or more of the generated fringe patterns comprises shifting by a fraction of the period of the fringe pattern.
 20. An intra-oral imaging apparatus comprising: a fringe pattern generator comprising a spatial light modulator and energizable to emit a fringe pattern illumination having a plurality of pattern features with a predetermined spatial frequency; a polarizer disposed in the path of the fringe pattern illumination emitted from the fringe pattern generator and having a first polarization transmission axis; a projection lens disposed to direct the polarized fringe pattern illumination as incident illumination toward a tooth surface; an imaging lens disposed to direct at least a portion of the light reflected and scattered from the incident illumination at the tooth surface along a detection path; an analyzer disposed along the detection path and having a second polarization transmission axis; a detector disposed to form image data from light that is obtained from the tooth along the detection path; a filter disposed along the detection path for transmitting light in a range from approximately 350-500 nm and attenuating light outside the range; and a control logic processor responsive to programmed instructions and actuable to obtain image data from the detector and to adjust the intensity of light over one or more portions of the pattern features of the fringe pattern illumination that is emitted from the fringe pattern generator according to the obtained image data. 